Petroleum drilling and production operations require large quantities of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several techniques.
In conventional oil well wireline logging, a probe or “sonde” housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
The problem with obtaining downhole measurements via wireline is that the drilling assembly must be removed from the drilled borehole before the desired borehole information can be obtained. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. In this situation, thousands of feet of tubing may need to be removed and stacked on the platform (if offshore). Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool can be an expensive proposition.
As a result, there has been an increased emphasis on the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. Designs for measuring conditions and formation properties downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “logging-while-drilling” techniques, or “LWD.”
When oil wells or other boreholes are being drilled, it is frequently necessary or desirable to determine the direction and inclination of the drill bit and downhole motor so that the assembly can be steered in the correct direction. Additionally, information may be required concerning the nature of the strata being drilled, such as the formation's resistivity, velocity, porosity, density and its measure of gamma radiation. It is also frequently desirable to know other downhole parameters, such as the temperature and the pressure at the base of the borehole, for example. Once this data is gathered at the bottom of the borehole, it is necessary to communicate it to the surface for use and analysis by the driller.
In LWD systems, sources and receivers are typically located at the lower end of the drill string. Typically, the downhole sources and receivers employed in LWD applications are positioned in a cylindrical drill section that is positioned close to the drill bit. As the drill bit progresses through the formation, drilling noise, the noncircular shape of the borehole, and the location of the logging tool in the borehole may effect the collection of formation data. Each of the sources may be programmed to generate a pure n-pole acoustic signal with a single mode of propagation. Thus, n=1 is a monopole acoustic signal with monopole mode of propagation, n=2 dipole acoustic signal with dipole mode of propagation, n=4 quadrupole acoustic signal, n=6 hexapole acoustic signal, and so on. Acoustic signals generated by the sources travel through the borehole, and along the borehole walls of the formation or into the formation depending on the velocity of the acoustic signal in the formation (Vf) and the velocity of the acoustic signal in the borehole (Vb). Each type of n-pole acoustic signal permits determination of different formation properties as described in more detail below. If the borehole is not circular, the tool is not in the center of the borehole, the sources are mismatched (i.e., sources given the same input do not generate identical acoustic signals), or the receivers are not balanced (i.e., receivers see identical acoustic signals at their inputs but generate varying electrical outputs for each signal), the signals at the receivers may have multiple modes of propagation (e.g., signal with both monopole mode and dipole modes of propagation). An acoustic signal with multiple modes of propagation arriving at the receivers interfere with each other and make the determination of formation properties inaccurate and difficult.
Thus, there is a continuing need for generating an n-pole acoustic signal with a single mode of propagation along the walls of the borehole that compensates for drilling noise, noncircular imperfections in the shape of the borehole, and the location of the logging tool in the borehole.